This invention relates generally to the field of electronically controlled compasses for use in an automobile. More specifically, it relates to the type of electronic compass which employs a flux-gate sensor similar to the type described in the article by Garner entitled "A Magnetic Heading Reference For the Electro Fluidic Auto Pilot" which appeared in two parts, one in the November 1981, and one in the December 1981 issues of Sport Aviation and are hereby incorporated by reference.
The theory on operation of flux-gate magnetic field sensors in an automotive application is further described in an article entitled "Magnetic Field Sensor And Its Application To Automobiles" by Hisatsugu Itoh dated February, 1980 and published by the Society of Automotive Engineers as "Paper No. 800123". This reference is also hereby incorporated by reference.
Further examples of work in the area of electronic compasses appears in the following U.S. Pat. Nos.: 3,943,763 issued Mar. 16, 1976 to Howell D. Garner entitled "Magnetic Heading Reference"; 4,424,631 issued Jan. 10, 1984 to Franks entitled "Electrical Compass"; 4,425,717 issued Jan. 17, 1984 to Marcus entitled "Vehicle Magnetic Sensor"; 4,505,054 issued Mar. 19, 1985 to Clark et al. entitled "Magnetic Sensor Mounting System"; and 4,546,551 issued Oct. 15, 1985 to Franks entitled "Electrical Control System."
U.S. Pat. No. 3,943,763 to Garner describes an electrically compensated magnetometer (A magnetometer's function is to detect magnetic fields and to measure their magnitude and direction.) It provides a means for compensating for the northerly turning error in a magnetometer used as a heading reference for a vehicle such as an aircraft. Northerly turning error is described in "A Magnetic Heading Reference For the Electro Fluid Auto Pilot", the articles cited above. Basically it arises from the fact that the earth's magnetic field is not horizontal except for a few locations. The lines of magnetic flux in the United States are between 60 to 75 degrees from the horizontal. This, according to Garner's Sport Aviation articles, is the dip angle.
U.S. Pat. No. 4,424,631 to Franks describes a circuit to work with an electrical compass. It features an oscillator coupled to a digital bandpass filter and a synchronous phase detector in such a way that the frequency and phase of output signals from a flux-gate sensor have a predetermined relationship to the excitation signal. According to Franks, this results in a tuning free design since the same oscillator signal is presented to excite the flux-gate sensor.
U.S. Pat. No. 4,425,717 to Marcus describes a flux-gate sensor mounting structure for an electrical compass in a vehicle. According to Marcus, the location of the sensor in association with the rearview mirror somewhat isolates the sensor from the metallic mass of the vehicle and from the vehicle's wiring.
U.S. Pat. No. 4,505,054 to Clark et al. describes another mounting system for a flux-gate sensor. According to Clark et al., this mounting structure includes a cradle for pivotally mounting the sensor for adjustable movement in a vertical plane aligned with the longitudinal axis of a vehicle.
U.S. Pat. No. 4,546,551 also to Franks describes an electrical control system, including a microcomputer for correction control of an electrical compass. The correction signal is applied directly to the sensor windings to drive the sensor output signal to the desired level. Northerly turning error is corrected via a signal processing procedure which affects the display output signals based on stored variation correction angle information.
An object of the present invention is to improve the systems of the prior art by supplying a simplified calibration technique utilizing the combination of software and hardware; the hardware being used to operate with voltage levels in required ranges and the software used to perform the precise calibration.
Another object of the present invention is to multiplex a bandpass filter circuit to work with the flux-gate sensor output to minimize the amount of electronic circuitry required to work with the control system and to reduce the effect of gain on the circuit. This is done by emphasizing repeatability.
A further object of the present invention is to provide an auto-ranging circuit to bring a calibration voltage into a narrow range to allow more precise compass calibration by means of software. This circuit also eliminates the need for an analog potentiometer and the associated calibration adjustments.
Still another object of the subject invention is to present a digital filter for smoothing out the magnetic field components prior to directional computation.
Yet another object of the present invention is to provide a microcomputer controlled quick ranging technique to quickly bring a voltage into a desired calibration range.
This application is one of three applications all filed on the same day, Dec. 27, 1985, and all commonly assigned having similar specifications and drawings; the other two applications being identified below:
______________________________________ U.S. Ser. No. Title Inventor ______________________________________ 814,125 The Multiplexing Of A Bandpass Ronald F. Hormel Filter Circuit To Work With A Flux-Gate Sensor Output 814,133 Microcomputer Controlled Quick William V. Luitje Ranging Technique and Digital Filter ______________________________________